A radio frequency (RF) power amplifier (PA) is a device for receiving an input RF signal having an input power and generating an output RF signal that has an output power which is larger than the input power. The input RF signal can be, e.g., an input current and/or an input voltage. The output RF signal can be, e.g., an output current and/or an output voltage. An RF PA may also be referred to simply as an amplifier. The power ratio of the output RF signal with respect to the input RF signal is commonly referred to as the power gain. An ideal amplifier has a power gain that is independent of the power of the input RF signal. In practice, however, an RF amplifier is not able to output an arbitrarily high power. The power gain of an amplifier therefore tends to become smaller with increasing input power. For example, the power gain may be limited by a maximal output current capability of the RF PA. The power domain of the input RF signal for which the power gain may be considered constant is known as the linear domain or linear regime. When the input power exceeds the linear domain, the amplifier is commonly said to be saturated or to work in the saturation regime.
Typically, RF PAs operational in the linear domain or linear regime, i.e. at low or intermediate input power levels, are less power efficient than RF PAs operational in the saturation regime, i.e. at high input power levels. A RF PA operational in the linear regime may have enhanced linearity but lower power efficiency than a PA operational in the saturation regime.
Techniques are described in literature to improve the power efficiency at low and intermediate input power levels while maintaining the power efficiency in the saturation regime. An example of such techniques is shown in U.S. Pat. No. 7,078,976 B2. U.S. Pat. No. 7,078,976 B2 describes an example of a Doherty power amplifier with a main amplifier stage and a peak amplifier stage both connected with an input power splitter at the input, and with an output power combiner at the output.
The output power combiner is integrated in one package by means of lumped capacitors and bond wires. However, integration of the output power combiner in the package may require that a large amount of layout space of the package is dedicated to the integration of the output power combiner, thereby increasing the costs of the package. Further, the package integrates large active dies, e.g. with gate periphery larger than 60 mm, and uses a wide output lead that generates parasitic distributed inductances and capacitances. As a consequence, a large impedance variation may be seen by the main amplifier stage along a lateral direction of the main amplifier stage, thereby resulting in a non-uniform loading of the main amplifier stage and poor Doherty performance.